An optical imaging system based on a coherence frequency domain reflectometry has become a powerful method of visualizing microstructural optical properties such as absorption, scattering, loss, birefringence, and a spectrum analysis, with a high resolution.
In a conventional optical imaging system, a carrier frequency of a continuous-wave laser beam is repeatedly linearly chirped with time. The linearly chirped laser beam is split into a reference beam and an interrogating beam projected onto an observed object to be recorded.
The interrogating beam reflected from the object is delayed, and thus a finite frequency difference occurs between the interrogating beam and the reference beam. Here, since a magnitude of the finite frequency difference is linearly proportional to a depth location of a cross-section of the object, the depth location of the cross-section of the object can be checked using the magnitude of the finite frequency difference.
For example, if a frequency chirp rate is “S Hz/sec,” and the reflected interrogating beam is delayed for time “τ,” the reflected interrogating beam and the reference beam interfere with each other in a photodetector and thus form an optical frequency difference “δf=S×τ.” Thus, “δf” is defined as a relative frequency difference between the reference beam and the interrogating beam. Also, a path difference corresponding to the frequency difference, i.e., the depth location “Δz” of the cross-section of the object, may be determined as “Δz=c×δf/2S,” wherein “c” denotes a velocity of light in a transmission medium.
Such an existing optical frequency domain reflectometry system detects a photodetector beat note and reads a bit frequency using an electronic spectrum analyzer and/or Fast Fourier Transformation (FFT) that is a high strength operation to detect a distance difference between the observation location and an observed depth location of the cross-section of the object, i.e., the depth location “Δz” and the reflectivities of the object.
For this purpose, the object is laterally scanned by the interrogating beam, and the depth location and the reflectivities are recoded according to each lateral scan position in order to obtain 3-dimensional image information of the object.
An optical coherence frequency domain reflectometry encodes information about a depth location and reflectivity of an object to be recorded in an optical frequency difference domain and thus does not require a high-speed electronic device that is used for a direct detection based on an optical range system. Also, according to the optical coherence frequency domain reflectometry, moving portions do not ideally exist, and thus a high-speed image can be potentially formed.
In the optical coherence frequency domain reflectometry, a depth resolution is determined by a photodetector used for measuring an optical beat note frequency and a chirp rate of a laser beam.
However, when spatial information of an object to be recorded is extracted, a conventional optical imaging system must collimate a focus of an interrogating beam using an objective lens to obtain a high lateral resolution. The depth range of an object to be recorded must be within a focus area of the interrogating beam having passed the objective lens, i.e, a Rayleigh range.
A spot size determines the lateral resolution and is proportional to a size of the focus area of the interrogating beam. In other words, the spot size of the interrogating beam and the focus area are determined as general Rayleigh limits. Here, the spot size “Δr” is “Δr=λ/2 sin(a/f),” the focus area “ZR” is “ZR=λ/sin2(a/f),” “λ” is a wavelength of a beam, “a” is a radius of the beam, and “f” is a focal length of the objective lens.
Accordingly, if the focus area “ZR” is increased, the lateral resolution is deteriorated. If the lateral resolution is increased, the focus area is shortened. For example, since a 3-dimensional microscope using a coherence frequency domain reflectometry requires a high resolution, a focus area is seriously limited. Thus, the time for scanning an object to be recorded is lengthened.
There is required a method of overcoming limits of such a conventional technique, obtaining a high lateral resolution in a long depth area, expanding a spot size, and increasing a focus area.